ackchuyally It wasn't illegal to own another person until 2010, you're confusing the year.
But everything else surrounding slavery was illegal, so they just sort of overlooked making it illegal to own someone. You couldn't work anyone like a slave, make them labour or do as you said without payment and fair hours, human trafficking wasn't allowed.
So basically the most you could do was own another person, if they entered into it willingly, and you couldn't actually treat them as a slave without it being illegal.